Surface Tension Seal

ABSTRACT

Present invention involves a seal means suitable to replace the classic seals. For instance the seal means of present invention can be a liquid seal ring ( 7 ) to replace the classic rubber seal rings. Such is liquid O-ring can be adapted to resists the actuation pressure relying on surface tension forces. Such liquid O-rings can also be linked in series, hereby increasing the maximum seal pressure. In a preferred embodiment the seal means of present invention comprises two major components, a surface tension seal ( 7 ) and a pressure divider ( 4 ). The pressure divider will comprise a system that generates a pressure drop. Such pressure divider can be located before the surface tension seal and perform the first pressure drop. The combination of these two systems has the advantage that the surface tension seal is able to reduce the leakage to zero or essentially zero and that the pressure divider is able to perform a large pressure drop. Nevertheless, the surface tension seal can also create a considerable pressure drop. This means that in applications where the seal pressure of the surface tension seal is not exceeded, the pressure divider can be omitted. However, omitting the pressure divider will always result in tighter tolerances when manufacturing the seal.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to a surface tension seal based on afluid, which is particularly suitable to seal hydraulic, pneumatic orhydro pneumatic attenuators. The invention also relates to actuators, inparticular nucroactuators, comprising at least one surface tension seal.The fluid seal can be used to seal a gas or a liquid. The surfacetension seal can be used as a dynamic seal or as a static seal. Aparticular technical advantage is that this seal is characterised by lowfriction and/or low leakage. The invention is applicable for both macroand micro devices but is particularly interesting for the latter.

BACKGROUND OF THE INVENTION

This invention relates to a fluid seal, preferably a liquid seal, toseal a gas or a liquid in for instance hydraulic cylinders, pneumaticcylinders or hydropneumatic cylinders. In an embodiment of presentinvention a seal has been described which can be divided in two majorcomponents. The first component will be referred to as the “surfacetension seal”, while the second will be referred to as the “pressuredivider”.

The operation principle of the surface tension seal is comparable tothat of sealing rubber O-rings but the seal ring is made of a fluidinstead of rubber. A liquid O-ring for instance resists the actuationpressure relying on surface tension forces. These O-rings can be linkedin series, hereby increasing the maximum seal pressure.

The pressure divider is a system that generates a pressure drop by meansof an hydraulic equivalent system to an electric voltage divider. Thepressure divider is located before the surface tension seal and performsthe first pressure drop. The main advantage of combining these twosystems is that the surface tension seal is able to reduce the leakageto zero and that the pressure divider is able to perform a largepressure drop. Nevertheless, the surface tension seal can also create aconsiderable pressure drop. This means that in applications where theseal pressure of the surface tension seal is not exceeded, the pressuredivider can be omitted. However, omitting the pressure divider willalways result in tighter tolerances when manufacturing the seal.

The presented seal technology is most interesting for micro devicessince surface tension is one of the strongest forces at microscale. Inaddition, the described seal is easy to miniaturize since no complexcomponents are needed. The presented seal offers for instance a solutionfor the sealing of pneumatic actuators, of hydropneumatic actuators orof hydraulic actuators and is in particular interesting for hydraulicmicroactuators since the maximum seal pressure scales inverselyproportional to size and the seal can be manufactured as one monolithicpart with the actuator cylinder.

An hydraulic actuator suitable for present invention can for instancehydraulic cylinders or hydraulic clamp cylinders as for instance madeavailable to the public by companies such as MICO, AHP Merkle, LJMHydraulik, Spx Fluid Power, Amf Andreas Maier, Atos, De-Sta-Co, DoucheHydro, Duplomatic, Oleodinamica, Duplomatic, Eaton—Hydro Line,Eaton—Vickers, Glual Hidraulica, Regnier, Rexroth—Industrial Hydraulics,Ljm Hydraulik. Hydraulic actuators suitable for present invention canalso be linear cylinders, hydraulic light-alloy cylinders, single-actingand double-acting hydraulic actuators, aluminum hydraulic cylinders,flat cylinders, hollow piston cylinders, telescopic hydrualic cylinders,hydraulic locking cylinders for injection molding machines, hydraulicpancake lock-nut cylinders, pancake lock nut cylinders, hydraulic rotaryactuators, rotary cylinders, hydraulic servo-cylinders, lock-nuthydraulic cylinders, as for instance been produced by Ds Dynatiec,Eckart, Olaer Industries, HKS, Montanhydraulik, Atos, Glual Hidraulica,Enerpac, Ljm Hydraulik and Hidraulica Um Plopeni. Also Pneumaticcylinders or hydropneumatic cylinders from Farger & Joosten or Eckart,water hydraulic cylinders from LJM Hydraulik and tie rod hydrauliccylinders from Eaton—Hydro Line or Universal Hydraulik andmini-hydraulic cylinders from Hunger Hydraulik ca be suitable forpresent invention.

The surface tension seal of present invention is particularly suitablefor microdevices. During the last decades several scientific studiesfocused on the choice of appropriate actuation technologies formicrodevices. Especially electrostatic and electromagnetic motors wereinvestigated. However, recent research shows that hydraulic systemsdevelop a higher force, work and power density at micro scale (BütefischS., et al. Sensors and actuators A, 2002, 638-645; Binnard, M., LegDesign for a Small Walking Robot, Master's thesis MIT, 1995, Wapner P.,Hoffman W., Sensors and actuators B, 2000, 52-60 and Peirs J., et al.,Sensors and Actuators A, Vol. 85, 2000, 409-417).

Microhydraulic systems have for instance been described in N. R. Tas, T.S. J. et al. Pages: 174-177 Nanotech 2002 Vol. 1 and fluidicmicrodevices have for instance been described in Harley, J. et al. 1989,Proc. IEEE pp. 25-28; Kricka, L, et al. 1989, “Liquid PrecisionEngineering and Optomechanics Vol. 1167, pp. 159-168 and Pfahler, J., etal. 1990, Sensors and Actuators, Vol. A21-A23, pp. 431-434 andmicropneumatic systems have for instance been described in Bütefisch Set al Sensors and actuators A, 2002, 638-645.

Despite these promising properties, hydraulic actuators are rare inMicro Systems Technology (MST) because of inherent sealing difficulties.Most existing miniature hydraulic actuators suffer from either leakageor high friction. These problems are due to the fact that the typicallyused rubber O-rings or lip seals are not appropriate for micro devices.The main technological barrier for the development of such microactuators is the lack of an appropriate sealing technology.

Thus, there is a need in the art for a novel for low friction seal. Thepresent invention solves the problems of the related art by replacingtraditional rubber O-rings for instance by a fluid O-rings, preferablyby a liquid O-ring. The latter resist the actuation pressure relying onsurface tension forces. A seal in accordance with this invention has anumber of advantages. Due to the use of a surface tension seal, thesolid-solid contact between the seal and the moving part can be avoided.Therefore, friction and seal wear can be reduced substantially. If theapplied pressure does not exceed its designed value, there will be noleakage. This is an important property for all applications that need ahermetic seal. Furthermore, this seal is easy to miniaturize and opensnew prospective for the sealing of fluidic microdevices. The technologyof present invention can also be used in any other applications where alow leakage, a low friction or both are desired as for instance inprecision mechanics.

Besides high seal pressures, an important advantage of this technologyare that the solid-solid contact between the seal and the moving partcan be avoided and consequently, friction and seal wear can be reducedsubstantially.

Typical applications for this microactuator are process automation,inspection and minimally invasive surgery.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION Description of the Invention

The invention will now be described by way of example and with referenceto the accompanying drawings.

FIG. 1 illustrates a possible configuration of the invention. In thisconfiguration the invention seals the piston (7) of a piston typeactuator. The surface tension seal (1) and the pressure divider (2-4)are integrated in the cylinder wall (8). The seal and cylinder can bemanufactured as one monolithic structure or can be assembled out ofdifferent components. The former approach can be advantageous for amicroactuator since microdevices are difficult to assemble.

In the case of FIGS. 1 and 2, the surface tension seal consists of anannular cavity (1,11), which is filled with a seal fluid. The cavity isfilled until the opposing surface (7) in FIG. 1 is reached, herebyseparating a fluid 1 at high pressure (P_(h)) from a fluid 2 at lowpressure (P_(l)). The invention can seal both liquid and gaseous fluids,and both over-pressures and under-pressures (eg. vacuum). The seal fluidhas preferably a high surface tension. For instance water, mercury,gallium or any other fluid that has a surface tension different fromzero can be used. It can be interesting to use eutectic alloys orundercooling to keep high surface tension materials liquid at theoperation temperature. For instance pure gallium steadily remainsundercooled at room temperature. This means that a gallium seal can beused below its melting temperature, for example at room temperature,without needing a heating element to keep the gallium in liquid state.

Besides mercury, water and gallium are promising sealing fluids. Themain advantages of water are that it is harmless and readily available.This makes water interesting for applications where strict materialregulations are imposed as in the food industry or for minimallyinvasive surgery. However, in other applications Ga is more attractivebecause of its excellent surface tension (γGa=0.7 N/m, γHg=0.5 N/m,γH2O=0.07 N/m). According to the directive 67/548/EEC, Ga is not adangerous product and therefore, it can be used in most applications.The main disadvantage of a Ga seal is that its melting point is justabove room temperature (303 K). Consequently Ga should be heated above303 K in order to be liquid and seal. However, pure Ga steadily remainsundercooled at room conditions (293 K, 1 atm) (Mellor J. W., Acomprehensive treatise on inorganic and theoretical chemistry, Longmans,Green and Co., London, 1924), and therefore, heating is not necessarilyrequired. Another way to keep Ga liquid at room conditions is to make aneutectic alloy of for instance Ga and In. Mixing about 20% weightpercent of In with Ga reduces their melting temperature to 15.3° C.

The maximum pressure drop ΔP=P_(h)−P_(l) that can be sealed isdetermined by the Laplace equation (i). This equation states that acurved fluid surface generates a pressure drop (ΔP) that is proportionalwith the surface tension (γ) of the fluid and is inversely proportionalto the two main radii of curvature (R₁, R₂). The dominant curvatureusually depends on the dimensions of the gap h and the contact angle α(see FIG. 2 and equation ii).

$\begin{matrix}{{\Delta \; P} = {\gamma \cdot \left( {\frac{1}{R_{1}} + \frac{1}{R_{2}}} \right)}} & (i) \\{R = \frac{h}{2\cos \; (\alpha)}} & ({ii})\end{matrix}$

Equations (i) and (ii) show that the seal pressure can be maximized byreducing the gap width (h) and the contact angle (α), as shown in FIG.2. Another way of maximizing the seal pressure is to increase thesurface tension (γ). Furthermore, the contact angle α, as shown in FIG.2, should be smaller than 90° for the seal to work.

The surface tension seal can be mounted in series as shown in FIG. 3. Byusing a series arrangement of seals, the total maximum seal pressure canbe increased while the seal fluid and the gap h (see FIG. 2) remain thesame. In order to have a good cooperation between the different seals,an appropriate compressibility and volume of the fluid between thedifferent seals must be chosen.

The seal fluid or liquid can be mounted in the seal cavity by anypreferred means. For instance a supply channel (10) that connects theseal cavity to a seal fluid or liquid reservoir can be used to force thefluid in the seal cavity. Alternatively, the seal fluid can be broughtin the cavity through the gap h (see FIG. 2). Another approach is tomount the seal in solid state and heat it above its melting point oncemounted. For example in the case of gallium, which has a melting pointjust above room temperature (around 30° C.), this can be an interestingapproach.

The seal can be anchored using a change in gap size, like in the case ofa seal cavity (see FIGS. 1-4 and equations i-ii), but can also beanchored using a change in wetting property (see FIG. 5-6) or any otherpreferred mean. In FIGS. 4, 5, and 6, (15) is a non-wetting surfacewhile (18) is a wetting surface. In the case of anchoring by a change inwetting property, the seal is kept in position by repulsive forcesexerted by non-wetting surfaces and/or attractive forces exerted bywetting surfaces. The use of a change in wetting property isparticularly interesting in applications where the two sealed parts donot move axially with respect to each other (eg. static and rotatingseals). In these applications, both sides of the fluid ring can generatea pressure drop as shown in FIG. 5.

Furthermore, it is possible to use the same fluid for the surfacetension seal as for the fluid to be sealed. An example of thisarrangement is shown in FIG. 6. An advantage of this approach is thatthese seals are easy to mount but the fact that the fluid to be sealedand the seal fluid must be the same can be a limitation. Here again, theseal can be anchored by a change in gap size, a change in wettingproperties or any other preferred mean as shown in FIG. 6.

In what follows, this application describes a pressure dividing systemthat allows the surface tension seals, as described above, to sealhigher pressures. This technology does not increase the wear or theleakage of the seal, and does not affect its hermetic properties. Theoperation of the pressure divider will be described by means of thepiston seal of FIG. 1. The pressure divider forces the sealed fluid toflow through a restriction (4) before it reaches the surface tensionseal (1) described above. A channel (2) is provided to allow the smallamount of fluid that passes through the restriction to flow back to forinstance a reservoir. A pressure rectifier for instance an annularcavity (3) can be added to the seal in order to avoid radialfluctuations in the pressure working on the surface tension seal.

The operation of the pressure divider can be understood by looking atthe simplified representation of FIG. 7 left. In this figure, thesurface tension seal (21) is represented by a valve that opens when theinlet pressure exceeds a threshold value, i.e. the maximum seal pressureof the surface tension seal. The leakage channel (20) is a representedby a (low) restriction while the pressure divider restriction (23) isrepresented as a (high) restriction. The restriction (23) and theleakage channel (20) are a hydraulic equivalent of a voltage divider,this means that they are able to divide the pressure working on thesurface tension seal.

If the flow resistance of the restriction can be made very large incomparison with the flow resistance of the leakage channel, the pressureworking on the surface tension seal can be reduced considerably. Forexample in FIG. 1 this can easily be achieved since the flow resistanceis very sensitive to the width of the clearance between the piston andthe cylinder in the restriction. In the case of a rotating seal the flowresistance of the restriction can be increased using a labyrinth (55),in the case of a static seal the flow resistance can be increased byusing a porous material (50) or any other preferred mean. The flow goingthrough the leakage channel can be collected and can be handled aspreferred. For instance, the leaking fluid can be accumulated in a localreservoir, can be channelled back to a low-pressure tube or can beevaporated. In the case of a hydraulic actuator, the leakage flow can bereturned to the low-pressure channel right before the valve as describedin FIG. 8. Furthermore, the leakage flow can be kept very low if theflow resistance of the restriction (23) and the leakage channel (20) ishigh enough. In applications where a low leakage through the seal isacceptable, the surface tension seal can be replaced by a restriction.

The main advantage of combining a surface tension seal with a pressuredivider is that the surface tension seal is able to reduce the leakageto zero and that the pressure divider is able to perform a largepressure drop. Nevertheless, the surface tension seal can also create aconsiderable pressure drop. This means that in applications where theseal pressure of the surface tension seal is not exceeded, the pressuredivider can be omitted. However, omitting the pressure divider willalways result in tighter tolerances when manufacturing the seal.

If needed, a pump can be added to the leakage channel (2) (63). This canreduce furthermore the pressure working on the surface tension seal (1,60), and thus enhances the maximum seal pressure. In many cases thispump can be integrated in a compact way in the structure to be sealed.For instance a venturi pump (61)-(64) can be integrated in the cylinderwall of a double working actuator as depicted in FIG. 12. The pump isrepresented schematically in dark grey. The pump and the cylinder can bemade as one monolithic part, like shown in FIG. 12 or can be made ofdifferent assembled components. FIGS. 12 a and 12 b show the samestructure but FIG. 12 b is a top view FIG. 12 a. In these figures, theinside of the actuator is shown in dashed lines, while the venturi pump(61)-(64) and the contour of the actuator is shown in solid line. Theventuri pump, consisting of two supply channels (61) (62), a narrowing(64) and a suction channel (63), can be fabricated by as preferred bythe user. FIG. 13 a shows a three dimensional section of the actuatorrepresented in FIG. 12. This section reveals the pump (69) (70) (71)located in the cylinder wall. In FIG. 13 b, the actuator is rotated over180° with respect to FIG. 13 a and a quarter of the actuator has beencut away in order to show its inside. Analogous configurations can beused for static, linear, rotational or other seal applications. Ifpreferred other pumping mechanisms can be used. The pump can also belocated downstream of the leakage channel (2), it can for instance belocated by the fluid reservoir (31).

DESCRIPTION OF CONFIGURATIONS AND FIGURES

The seal of present can be used in many different configurations. Inthis section, some configuration examples will be described by mean offigures. Most figures show a cross sections of the invention, thesecross sections can be made for example from a cylindrical or beam shapedstructure.

Referring now specifically to the drawings, a cross section of ahydraulic piston-type actuator, which is sealed using a single surfacetension seal (1) in combination with the pressure divider (2-4)according to an embodiment of the present invention is illustrated inFIG. 1:

-   (1) Surface tension seal-   (2) Leakage channel-   (3) Pressure rectifier-   (4) Restriction-   (5) Actuator pressure supply-   (6) Pressure chamber-   (7) Piston rod-   (8) Cylinder-   (9) Piston

FIG. 2 shows a cross section of the surface tension seal only. The sealis installed using a supply channel (10).

-   (10) Seal fluid supply channel-   (11) Seal cavity-   (12) High pressure chamber

FIG. 3 sketches a series arrangement of surface tension seals (13)mounted between two parts (14). This arrangement allows enhancing themaximum sealed pressure of the seal.

-   (13) Side walls-   (14) Surface tension seals

FIG. 4-6 show different anchoring mechanisms for the surface tensionseal. FIG. 5 is based on a change in gap, while FIGS. 5 and 6 use achange in wettability of the surfaces.

In FIG. 6, the surface tension seal fluid is the same fluid as the fluidto be sealed.

-   (15) Non-wetting surface-   (16) Seal fluid-   (17) High pressure side-   (18) Wetting surface-   (19) Low pressure side

FIG. 7 shows a simplified representation of the surface tension seal andthe pressure divider, as discussed in the description of the invention.

-   (20) Leakage channel-   (21) Surface tension seal-   (22) Pressure rectifier-   (23) Restriction

FIG. 8 gives an example of the outline of a complete hydraulic system.In this example the seal is used on a hydraulic actuator as shown inFIG. 1. The leakage flow is channelled to the low-pressure tube (29)right before the valve (27). A similar outline can be drawn for manyother applications.

-   (24) Piston-type hydraulic actuator-   (25) Driving channels of the actuator-   (26) Leakage channel-   (27) Valve-   (28) High pressure channel-   (29) Low pressure channel-   (30) Pump-   (31) Fuid reservoir

FIG. 9 shows how a seal in accordance with the invention can be used toseal the piston of a fluidic actuator. In the sketched cross section,the leakage flow is channelled trough the piston to the outside world.This approach is especially interesting in the case of pneumaticactuators because the leakage of air to the outside world of air isusually less detrimental then that of a liquid. In the case of ahydraulic actuator the fluid that flows through the piston can becollected by a dedicated system. Another possibility is to omit thepressure divider and use the surface tension seal only. A seriesarrangement as sketched in FIG. 3 can be interesting to increase theseal pressure.

-   (32) Surface tension seal-   (33) Leakage channel-   (34) Pressure rectifier-   (35) Restriction-   (36) Actuator pressure supply-   (37) Pressure chamber-   (38) Cylinder-   (39) Leakage channel-   (40) Piston-   (41) Restriction-   (42) Check valve-   (43) Surface tension seal-   (44) Pressure rectifier-   (45) Pressure chamber

FIG. 10 shows a cross-section of a static seal in accordance with theinvention. An advantage of this setup is that for example a needle canbe inserted trough the surface tension seal, for example to sample thesealed fluid. In this figure the restriction of the pressure divider isa porous material and the surface tension seal is anchored by a changein wetting property in accordance with FIG. 5.

-   (46) Needle-   (47) Surface tension seal-   (48) Leakage channel-   (49) Pressure rectifier-   (50) Restriction-   (51) High pressure vessel-   (52) Vessel wall

Picture 11 shows a rotating configuration of the seal in accordance withthe invention. In this example, the restriction of the pressure divideris generated by a labyrinth (55).

-   (53) Pressurized fluid-   (54) Rotator-   (55) Labyrinth restriction-   (56) Surface tension seal-   (57) Stator-   (58) Pressure rectifier-   (59) Leakage channel

FIG. 12 shows a section of a linear actuator with a venturi pumpintegrated in the actuator wall. The pump is represented schematicallyin dark grey. FIGS. 12 a and 12 b show the same structure but FIG. 12 bis a top view of FIG. 12 a. In these figures, the inside of the actuatoris shown in dashed lines, while the venturi pump (61)-(64) and thecontour of the actuator is shown in solid line. The venturi pump,consisting of two supply channels (61) (62), a narrowing (64) and asuction channel (63), can be fabricated by as preferred by the user. Thearrows in FIG. 12 b show the fluid or liquid flows. Similar structurescan be used for rotating or static seal applications in combination withall the described anchoring mechanisms.

-   (60) Surface tension seal-   (61) Pump supply channel in-   (62) Pump supply channel out-   (63) Suction channel-   (64) Venturi narrowing-   (65) Restriction-   (66) Cylinder-   (67) Piston

FIG. 13 shows a three dimensional sketch of a linear hydraulic actuatorthat is sealed using a surface tension seal, a pressure divider and apump, in accordance with the invention. This sketch shows a crosssection of the cylinder, hereby revealing the venturi pump that isintegrated in the cylinder wall. In FIG. 13 b, the actuator is rotatedover 1800 with respect to FIG. 13 a and a quarter of the actuator hasbeen cut away in order to show its inside.

-   (68) Cylinder-   (69) Pump supply channel-   (70) Venturi narrowing-   (71) Suction channel-   (72) Restriction-   (73) Surface tension seal cavity-   (74) Cylinder wall

The configurations discussed above are not exhaustive; the describedseal can be used in many other applications and configurations.

EXAMPLES Example 1 Example of a Fabrication Process

Intended as exemplary only a cylinder and seal was fabricated as onemonolithic structure. This minimizes the number of components thussimplifying the assembly of the actuator. The cylinder and the seal weremade of brass. The fabrication of the actuator was divided in 8 steps asoutlined in FIG. 14. In the first two steps, drilling and reaming withminiature tools defined the rough actuator shape. In step three thedimensions and the surface quality of the cylinder were improved byreaming. A tight tolerance on the seal diameter was achieved withplanetary Micro Electro Discharge Machining (μLEDM) in step 4. Electrodewear was compensated by Wire Electro-Discharge Grinding (WEDG) (Song X.et al. Proc. of SPIE Symposium on Micromachining and Microfabrication,Paris, 1999, 792-799). Next, a WEDG formed electrode was used to makethe seal cavity. In step 6, a connection channel was drilled using μEDM.Finally, a Ga supply channel was glued with low viscosity epoxy glue ona flattened surface (step 7 and 8). The bore of the actuator was 2 mmand the length is 22 mm.

An actuator with water or gallium as sealing fluid had successfully beentested up to 100 kPa. The actuator had a length of 22 mm, a bore of 2mm, and a stroke of 20 mm and developed a force up to 0.2 N withoutleakage.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein.

1-38. (canceled)
 39. An actuator comprising at least one fluid sealdesigned to separate fluids under different pressure, the actuatorfurther comprising a pressure chamber, wherein the fluid seal comprisesa surface tension seal (1, 14, 16, 21, 32, 44, 47, 56) and a pressuredivider between the surface tension seal (1, 14, 16, 21, 32, 44, 47, 56)and a pressure chamber (6, 12, 37, 45, 51), the pressure dividercomprising at least one leakage channel (2, 20, 33) and at least onepressure restriction (4, 23, 35) designed to prevent leakage at thesurface tension seal.
 40. The actuator according to claim 39, whereinthe pressure divider comprises a restriction (4, 23) between the surfacetension seal (1, 21) and pressure chamber (6) and said pressure dividerforces the sealed fluid to flow through the restriction (4, 23) beforeit reaches the surface tension seal (1, 21).
 41. The actuator accordingto claim 39, wherein the pressure divider further comprises a pressurerectifier.
 42. The actuator according to claim 41, wherein the actuatorfurther comprises a piston (9, 40) or piston rod (7), and the pressurerectifier is an annular cavity (3) between a leakage channel (2, 20) andthe piston (9, 40) or the piston rod (7).
 43. The actuator according toclaim 41, wherein the pressure divider further comprises a restriction(4) between the pressure rectifier (3) and the pressure chamber (6). 44.The actuator according to claim 39, including a porous material (50),wherein the resistance of the pressure restriction (35) is increased bythe porous material (50), and wherein fluids passing said pressurerestriction are forced to pass through said porous material (50). 45.The actuator according to claim 39, including a labyrinth, wherein theresistance of the pressure restriction (35) is increased by thelabyrinth (55), wherein fluids passing said pressure restriction areforced to pass through the labyrinth (55).
 46. The actuator according toclaim 39, including a pump (62) associated with the leakage channel (2,63) and arranged to further reduce pressure on the surface tension seal.47. The actuator according to claim 46, wherein the actuator is acylinder actuator having a cylinder wall, and the pump is a venturi pump(61-64) integrated in the cylinder wall of the cylinder actuator. 48.The actuator according to claim 47, wherein the pump and cylinder aremonolithic.
 49. The actuator according to claim 39, wherein the leakagechannel (2, 20) is provided to allow the small amount of fluid thatpasses through the restriction to flow back.
 50. The actuator accordingto claim 39, the actuator further comprising a collecting means tocollect fluid from the leakage channel (2, 20).
 51. The actuatoraccording to claim 39, the actuator further comprising a means tochannel fluid from the leakage channel (2, 20) back to the side of lowerpressure or to a low-pressure tube.
 52. The actuator according to claim39, wherein the pressure divider comprises an annular cavity (3) at theinner end of the channel (2) that is arranged to avoid radialfluctuations in the pressure working on the surface tension seal. 53.The actuator according to claim 39, wherein the surface tension seal isadapted to resist said pressure based on surface tension and to separatea fluid under actuating pressure (17) from a fluid under environmentalpressure (19).
 54. The actuator according to claim 39, wherein thesurface tension seal (1, 14, 16, 21, 32, 44, 47, 56) comprises a sealcavity (11) filled with a sealing fluid separating the actuator pressure(Ph) from the environmental pressure (Pl).
 55. The actuator according toclaim 54, wherein the seal cavity (11) is connected to a seal fluidsupply channel (10) to supply sealing fluid to said cavity.
 56. Theactuator according to claim 54, wherein the actuator is a piston drivenactuator comprising a piston (9, 40) or a round piston rod (7), andwherein the seal cavity is an annular cavity around the piston (9, 40)or around the piston rod (7).
 57. The actuator according to claim 39,wherein the surface tension seal (1, 14, 16, 21, 32, 44, 47, 56)comprises at least one fluid-O-ring.
 58. The actuator according to claim39, comprising an assembly of surface tension seals (1, 14, 16, 21, 32,44, 47, 56) mounted in series to increase the maximum seal pressure. 59.The actuator according to claim 39, wherein the sealing fluid comprisesa component selected from the group consisting of water (H202), mercury(HG) and gallium (Ga).
 60. The actuator according to claim 39, whereinthe sealing fluid is an eutectic alloy.
 61. The actuator according toclaim 39, wherein the sealing fluid is under cooled.
 62. The actuatoraccording to claim 39, wherein the sealing fluid comprises a galliumalloy.
 63. The actuator according to claim 39 wherein the actuatorcomprises a cylinder having a cylinder wall comprising at least onesurface tension seal (1, 14, 16, 21, 32, 44, 47, 56) for sealing apiston (9, 40) or a piston rod (7) and wherein the surface tension seal(1, 14, 16, 21, 32, 44, 47, 56) and the pressure divider (2-4) areintegrated in the cylinder wall (8, 38).
 64. The actuator according toclaim 63, wherein the cylinder and the surface tension seal is amonolithic structure.
 65. The actuator according to claim 39, whereinthe actuator is a linear actuator.
 66. The actuator according to claim39, wherein the actuator is a hydraulic actuator.
 67. The actuatoraccording to claim 39, wherein the actuator is a pneumatic actuator. 68.The actuator according to claim 39, wherein the actuator is a hydropneumatic actuator.
 69. The actuator according to claim 39, wherein theactuator is a micro hydraulic actuator.
 70. The actuator according toclaim 39, wherein the actuator is a micro pneumatic actuator.
 71. Theactuator according to claim 39, wherein the actuator is a micro hydropneumatic actuator.
 72. The actuator according to claim 39, wherein thesurface tension seal is anchored in the cylindrical actuator by contactwith an inner annular wetting surface (18) zone in the inner non wettingsurface of the cylinder.
 73. The actuator according to claim 39, whereinthe fluid seal is rotational.
 74. The actuator according to claim 39,wherein the fluid seal is static.
 75. The actuator according to claim39, wherein the fluid seal is linear.